1. Field of the Invention
The present invention relates to an FM radio receiver and a signal processing device used therein, and particularly to an FM radio receiver having a function of receiving a multiplex FM broadcast signal, on which data, such as character information, are superimposed.
2. Description of the Prior Art
Multiplex FM broadcasts, on which character information is superimposed, have recently begun to be transmitted. In this type of multiplex FM broadcasting, encoded character information is superimposed on stereo FM signals. The superimposed information includes news and traffic information at present. The broadcasting of figure information useful for an automobile navigation system is being investigated.
In the data radio channel (DARC) standard, multiplex FM data superimposed on a stereo FM signal have a frame structure composed of 288.times.272 bits, as shown in FIG. 1.
One line of the frame (one block) consists of a block identification block (BIC) composed of 16 bits, an error correcting code for correcting data errors which is composed of a cyclic redundancy code (CRC; 14 bits) and parity (82 bits), and a data packet composed of 176 bits.
The data packet in the block consists of a prefix composed of 32 bits and a data block composed of 144 bits, as shown in FIG. 2.
The respective superimposed FM data provided in multiplex FM broadcasting, such as character information, figure information and traffic information for an automobile navigation system, are assigned to one block of the transmitted frame. A service identification code is added to the prefix of the block so that it can be discerned what type of data among character information, figure information, traffic information for an automobile navigation system and so on, is superimposed on a received block.
In normal FM broadcasting, carrier waves are modulated by the main channel signal (L+R), pilot signal and sub channel signal (L-R). In multiplex FM broadcasting, a frequency band of the modulation signal for the superimposed FM data is set to be higher than that of the sub signal in normal FM broadcasting (centered at 76 kHz), in order not to interfere with the channel and pilot signals. Digital data obtained by level controlled minimum shift keying (LMSK) modulation in the higher frequency band is superimposed at a rate of 16 kbits/sec so that the block is transmitted for 18 msec, and accordingly, the frame is transmitted for 5 sec.
When receiving such multiplex FM broadcasts, a decoder for selecting and decoding the superimposed FM data, and a display for indicating character or figure information according to the provided service need to be added to a normal FM receiver.
In the multiplex FM broadcasting, a plurality of broadcasting stations in one group probably transmit an identical program at the same time, like the stations A and B shown in FIG. 3. When the program is received by a receiver placed in a moving body, such as a vehicle, it is possible that the intensity of an electric field of the carrier waves from the broadcasting station A will become smaller, while the intensity of the electric field of the carrier waves from the station B will become large. If the FM transmissions of the superimposed FM data are synchronized between the stations A and B in the group, as shown in FIG. 3, the receiver placed in the moving body can continuously receive the superimposed FM data by changing the station from A to B.
When the station is changed from A to B, a user first operates a selecting key, as in a normal receiver. The tuned frequency (i.e. the frequency signal) is changed at the front end of a multiplex FM receiver in response to the user's key operation, enabling the receiver to tune to the station B.
When changing the broadcasting station, it is necessary to change the tuned frequency from that of station A to that of station B, to realize the receiving of a broadcast from station B. It takes a slightly longer time to enable tuning of a broadcast of station B. The broadcast cannot be caught during this time, and consequently the data superimposed on the broadcast at this time fail to be correctly received, causing an error (NG).
The change in frequency signal is performed in response to a request to change the station through the operation of a selecting key at an arbitrary time, as described above. Therefore, even if the NG period for the data is 18 msec or less, as shown in FIG. 4A, the change in frequency signal can interrupt data of one block length, causing NG for data of two blocks.
When NG occurs for data of a plurality of blocks, it is difficult to correctly decode data superimposed on the frame. This leads to a problem that the previously received data becomes useless. Therefore, the reduction of useless data has been required.